THE SHOCK STRUCTURE IN THE PROTOPLANETARY NEBULA M-1-92 - IMAGING OF ATOMIC AND H-2 LINE EMISSION

We present HST imaging of continuum (5500 Angstrom) and atomic line (H alpha, [OI] 6300 Angstrom, [SII] 6717 and 6731 Angstrom, and [OIII] 5 007 Angstrom) emissions in the protoplanetary nebula M 1-92. Ground ba sed imaging of 2 mu m continuum and H-2 ro-vibrational (S(1) v=1-0 and v=2-1 lines) emission has been also performed. The 5500 Angstrom cont inuum is due to scattering of the stellar light by grains in a double- lobed structure comparable in extent and total density with the molecu lar envelope detected at mm wavelengths, which consists of two empty s hells with a clear axis of symmetry. On the other hand, the optical li ne emission comes mainly from two chains of shocked knots placed along the symmetry axis of the nebula and inside those cavities, for which relatively high excitation is deduced (shock velocities of about 200 k m s(-1)). The H-2 emission probably comes from more extended regions w ith representative temperature and density of 1600 K and 6 10(3) cm(-3 ), intermediate in location and excitation between the atomic line kno ts and the very cold region detected in CO emission. We argue that the chains of knots emitting in atomic lines correspond to shocks taking place in the post-AGE bipolar flow. The models for interstellar Herbig -Haro objects seem to agree with the observations, at least qualitativ ely, explaining in particular that the atomic emission from the bipola r flow dominates over that from shocks propagating in the AGE shell. M odels developed for protoplanetary nebula dynamics fair, however, to e xplain the strong concentration of the atomic emission along the symme try axis.